Process for the continuous manufacture of substantially acetic acidcontaining mixtures of low molecular weight fatty acids by paraffin oxidation



Aug. 8, 1967 K SENNEWALD ET AL 3,335,160

PROCESS FOR THE CONTINUOUS MANUFACTURE OF SUBSTANTIALLY ACETIC ACID-CONTAINING MIXTURES OF LOW MOLECULAR WEIGHT FATTY ACIDS BY PARAFFIN OXIDATION Filed April 17, 1962 MWEA/rOPJ It um? Semewald Wilhelm Uqyi H2 in Z fwyenfiacic Herbert fiwst Unite The present invention relates to a process for the manufacture of substantially acetic acid-containing mixtures of saturated fatty acids of low molecular Weight by oxidation of paraflinic hydrocarbons having more than 2, preferably 4 to 8 carbon atoms, or mixtures thereof in the liquid phase at an elevated temperature and under pressure with oxygen or oxygen-containing gas mixtures. The invention also provides an apparatus suitable for use in carrying out the process.

It is known that parafiinic hydrocarbons can be oxidized in the liquid phase with oxygen or an oxygen-containing gas, advantageously air, to fatty acids of low molecular Weight. The starting materials used include, for example, mixtures of hydrocarbons having 4 to 8 carbon atoms which consist substantially of straight chained and branched paraflins, that may include a small proportion of cyclo-parafiins and aromatic hydrocarbons, and which can be obtained by distillation of petroleum. The processes hitherto proposed and used industrially are carried out at an elevated temperature in the presence or absence of a catalyst in cylindrical high pressure reactors, the reaction mixture being sometimes moderately cycled by using some type of a thermo-siphon or a mammoth pump. These means are intended to ensure that the liquid reaction mixture admixes with oxygen or air. In spite of all these measures, the yield of fatty acid, calculated on the amount of carbon used, is moderate, While the space time yield is unsatisfactory since due to unsatisfactory mixing the reaction and residence times in the reactor are too long and the reaction space available is not utilized efficiently enough.

We have now found that the yield of fatty acid, calculated on the amount of carbon used, and more especially the space-time yield of fatty acid obtained, can be substantially improved in the manufacture of substantially acetic acid-containing mixtures of saturated fatty acids of low molecular weight by oxidation of parafiinic hydrocarbons having more than 2, preferably 4 to 8 carbon atoms, or mixtures thereof in the liquid phase at an elevated temperature and under pressure with oxygen or oxygen-containing gas mixtures. In accordance with this invention the mixture of hydrocarbons, their oxidation products and oxygen-containing gas is circulated through a flow reactor with the aid of pumps at a velocity of flow of at least 50 cm./sec., the circulated reaction mixture is separated into liquid and off-gas and a portion of the liquid is continuously Withdrawn from the cycle in known manner and processed by distillation. The first runnings and last runnings of the fatty acid fraction admixed with fresh hydrocarbon are introduced again into the cycle.

The apparatus used for carrying out the present process comprises a flow reactor which may be composed of a plurality of straight, series-connected pipe sections or have the form of a tubular coil. The tubes may be provided in the interior with disturbing means, if desired, in order to still improve the intended intense turbulent mixing of gas with liquid. At a relatively high velocity of flow, no such States Patent auxiliary means are necessary to produce such intense and turbulent mixing. The reaction tubes are jacketed; the jacket is charged with condensed steam maintained under pressure whereby the desired temperatures can be produced, i.e., by heating to initiate the reaction or by cooling to dissipate the reaction heat. The steam generated may also be used in other zones of the apparatus.

It is a further characteristic feature of the process of this invention that the residence time of the reaction mixture consisting of the hydrocarbons, their oxidation prodnets and oxygen containing gas, in the flow reactor varies, per passage, between 10 and seconds, advantageously 20 and 50 seconds.

The mixture leaving the flow reactor is conveyed to a separator from which the off-gas is removed through a gas cooler. The proportion of hydrocarbons included in the off-gas are recovered by washing with diesel oil. In the separator, wherein the reaction liquid must rapidly be circulated in order to produce turbulent mixing, the mixture is not separated into two liquid phases. A portion of the reaction mixture in the separator is branched off for distillation. At the same time, a corresponding proportion of mixture to be oxidized is introduced into the reactor so that the level of liquid in the separator is kept constant.

The portion continuously branched off the cycled mixture for being worked up amounts to 1 to 15% by volume, preferably 4 to 10% by volume, of the total quantity of liquid circulated per hour by pumping.

The preferred oxygen-containing gas is air, which may be enriched with oxygen as desired. Mixtures of oxygen with inert gases may also be used. The gas is used in a proportion such that the off-gas, after removal of all constituents condensable up to 20 C. and after the diesel oil wash, contains a maximum of 6%, advantageously 2 to 3%, oxygen.

The oxidation is carried out under a pressure of 40 to 60 atmospheres, preferably 45 to 50 atmospheres. Higher or lower pressures do not remarkably influence the course of the reaction.

The optimum temperature to be used for carrying out the oxidation varies between about and 200 C., advantageously and 180 C. Higher temperatures favor for the same residence time the formation of carbon dioxide, lower temperatures retard the velocity of oxidation.

The process of this invention yields more than 20% more fatty acid or more than 10% more fatty acid, calculated on the carbon used, and a 4 to 6 times greater space-time yield as compared with conventional processes and own tests carried out in a cylindrical reactor with and Without thermo-siphon circulation.

The process of this invention involves, inter alia, the advantage that no residues are obtained.

An apparatus suitable for use in carrying out the invention is shown diagrammatically. In the drawing, FIG. 1 represents the flow reactor and hence the oxidation Zone comprising several vertical tubes connected in series which are jacketed to receive the condensed steam which is used for heating or cooling.

The oxygen-containing gas is supplied through line 2 and the fresh hydrocarbon is supplied through line 3, the two components being introduced jointly through line 4 into flow reactor 1. By means of circulating pump 5 so great a proportion of reaction mixture is circulated through reactor 1 and separator 6 that the velocity of flow of the gas-liquid mixture is at least 50 cm./sec.; the difference in pressure between the inlet and outlet of the flow reactor is about 2 to 10 atmospheres absolute, preferably 4 to 6 atmospheres absolute. The mixture circulated is also introduced into the reactor through line 4. The residence time for one single passage through the fiow reactor amounts to 10 to 100 seconds, preferably 20 to 50 seconds. The reaction mixture leaving flow reactor 1 travels into separator 6 from which the off-gas is removed through a gas cooler 7. The off-gas freed from all products condensable up to 20 C. contains especially carbon monoxide, carbon dioxide and nitrogen when air was used as oxidizing agent. The off-gas is then subjected still under pressure to the diesel oil wash at 8 where lowboiling oxidation products and hydrocarbons are absorbed.

The off-gas is then released at 13. The diesel oil is freed in stripping column 9 from these low-boiling constituents which are returnedto the reactor through lines 10 and 3 into which a cooler is inserted. A part of the reaction mixture obtained in separator 6 is removed through line 11 for distillation. The same volume of mixture to be oxidized is introduced into the flow reactor through line 3 so that the liquid in separator 6 is maintained at a constant level.

The reaction product withdrawn is processed by distillation in a manner such that the constituents included in the reaction mixture which boil at a temperature lower than the C toC -acids and the higher boiling residue are returned into the reactor through lines 12 and 3. The C to C -acids are the desired reaction product. Residual products are not obtained in the process.

The considerable amount ofheat evolved during the oxidation can be dissipated partially by direct cooling. Another portion of the reaction heat is dissipated by evaporation of the unreacted hydrocarbon and the products formed during oxidation. This amount of heat appears in off-gas condenser 7.

The following example comprising 4 tests serves to illustrate the process and apparatus of this invention. In test 4 a conventional process was used for comparison.

Example A mixture of 48% n-parafimic hydrocarbons and 48.4% isoparafiinic hydrocarbons was used as starting material. The composition of the mixture in weight percent was as follows:

n-butane 1.0 n-pentane 17.3 i-pentane 10.9 n-hexane 29.0 2,2-dimethylbutane 0.5 2,3-dimethylbutane 15.9 B-methylpentane 13.0 n-heptane 0.7 2,4-dimethylpentane 0.5 2,2,3-trimethylbutane 5.9 2,3-dimethylpentane 0.8 B-methylhexane 0.9 Benzene 1.1 Other ingredients 2.3

The mixture had a mean carbon content of 84%, corresponding to a C-number of 7 per 100 g. hydrocarbon.-

The flow reactor 1 consisted of 10 stainless steel tubes having an internal diameter of 9 mm. and a length of 3.1 m., corresponding to a total capacity of 1.97 liter. The air and liquid admission lines 4 were disposed at the foot of the first tube. The reactor was continuously supplied with fresh hydrocarbon through line 3 and with air through line 2. The oxidation was carried out at 170 C. under a pressure of 45 kg./cm. Both the gaseous and liquid products were conveyed from the oxidation zone into separator 6 in which the gas was separated from the liquid. By means of circulating pump 5, the amount of liquid withdrawn from separator 6 and circulated through the flow reactor was maintained constant. .The off-gas travelled from separator 6 through gas cooler 7 to the diesel oilrwash 8 where itwas freed at 20 C. by washing with 2 liters/hour diesel oil from low-boiling organic constituents and then released at 13. The products absorbed in the diesel oil were recovered in a stripping column 9 at a sump temperature of 180 C. and

returned to the reactor.

In control test 4, the flow reactor was replaced by a Test 1 2 3 4 Velocity of flow (cm./sec.):

Gas 52.5 54.5 38 086 Gas liquid 96 83 5s Difierence in pressure before and ter reactor (absolute atmospheres) 5. 5 5.0 4.0 Residence time in reactor (seconds per passage) 32 37' 54 Amount of reaction mixture circulated (liters/hour) 100 75 45 Ofi-gas normal cubic meters/hour measured at N.l.P 2.58 2. 63 2.0 3. 55 Consumption of hydrocarbon (g./hour).. 405 455 331 1,136 Yield in' grams per 100 g. hydrocarbon (yield in percent on carbon used):

Formic acid 27. 7 28. 6 28. 4 25. 0

(8. 6) (8.9) (8. 8) (7. 5) Acetic acid 103. 5 106. 8 100. z. 77. 3

(49. 4) (50. 9) (50. 7) (36. 8) Propionic acid 3.0 3. 8 3.0 4. 2

(l. 7) (1. 8 (l. 8) (4. 8) Space-time yield:

Grams acetic acid 213 47 7 9 8 Liter reaction space-hour *6 hours, 38 minutes.

We claim:

1. A process for the manufacture of mixtures of saturated C fatty acids, the main constituent of said mixtures being acetic acid, which comprises passing in the liquid phase a mixture of parafiinic hydrocarbons having 4 to 8 carbon atoms, their oxidation products and oxygen-containing gas through a tubular flow reactor with a velocity of flow of at least 50 cm./sec., at a residence time of between 10 and 100 seconds per passage through said reactor, at a temperature between and 200 C; and under a pressure between about 30 and 80 atmospheres, continuously separating the crude reaction mixture leaving said tubular flow reactor into liquid gas, recycling a portion of said liquid to said flow reactor, distilling the remainder of said liquid to separate the C fatty acids therefrom and recycling the first and last runnings of the distillation to said flow reactor admixed with fresh parafiinic hydrocarbons having 4 to 8 carbon atoms.

2. The process of-claim 1 wherein the portion withdrawncontinuously from the recycled liquid for being worked up amounts to about 1 to 15% by volume per hour, calculated on the whole amount of liquid circulated per hour by pumping.

3. The process of, claim 1 wherein the amount of oxygen-containing gassupplied to the reactor is so selected that the ofigas, after removal of all constituents condensable at a temperature of up to 20 C. and after diesel oil wash, contains a maximum of 6% by volume oxygen.

4. The process of claim 1 wherein the separated C fatty acid fraction is a mixture consisting of formic acid, acetic acid and propionic acid.

5. The process of claim 1, wherein said residence time of said mixture is between about 20 and.50 seconds per passage through said reactor.

(References on following page) and off- I References Cited UNITED STATES PATENTS Chapman et a1 260-451 Mitchell et a1. 360-451 Nelson 360-451 Elce et a1. 260-533 Habeshaw et a1. 260-451 X Millidge et a1. 260-533 Armstrong et a1. 260-451 X 2,926,191 2/1960 Lawson-Hall et a1. 260-533 2,929,690 3/ 1960 Bennet et a1. 23-26 3 3,057,699 10/ 1962 Allbright et a1. 23-263 FOREIGN PATENTS 757,309 9/1956 Great Britain.

LEON ZITVER, Primary Examiner. B. HELFIN, H. T. MARS, Assistant Examiners. 

1. A PROCESS FOR THE MANUFACTURE OF MIXTURES OF SATURATED C1-3 FATTY ACIDS, THE MAIN CONSTITUENT OF SAID MIXTURES BEING ACETIC ACID, WHICH COMPRISES PASSING IN THE LIQUID PHASE A MIXTURE OF PARAFFINIC HYDROCARBONS HAVING 4 TO 8 CARBON ATOMS, THEIR OXIDATION PRODUCTS AND OXYGEN-CONTAINING GAS THROUGH A TUBULAR FLOW REACTOR WITH A VELOCITY OF FLOW OF AT LEAST 50 CM./SEC., AT A RESIDENCE TIME OF BETWEEN 10 AND 100 SECONDS PER PASSAGE THROUGH SAID REACTOR, AT A TEMPERATURE BETWEEN 140* AND 200*C. AND UNDER A PRESSURE BETWEEN ABOUT 30 AND 80 ATMOSPHERES, CONTINUOUSLY SEPARATING THE CRUDE REACTION MIXTURE LEAVING SAID TUBULAR FLOW REACTOR INTO LIQUID AND OFFGAS, RECYCLING A PORTION OF SAID LIQUID TO SAID FLOW REACTOR, DISTILLING THE REMAINDER OF SAID LIQUID TO SEPARATE THE C1-3 FATTY ACIDS THEREFROM AND RECYCLING THE FIRST AND LAST RUNNINGS OF THE DISTILLATION TO SAID FLOW REACTOR ADMIXED WITH FRESH PARAFFINIC HYDROCARBONS HAVING 4 TO 8 CARBON ATOMS. 